Silicon is a basic material in the primary industry, namely the electronic information technology industry throughout the world and accounts for over 95% of the use amount of semiconductor materials in the world. Zone-melted silicon single crystals have the advantages of high purity, good uniformity, few defects and the like owning to its unique growth mode and are thus applicable to high-power semiconductor components. With vigorous development of the electric and electronic industry, various novel electric and electronic devices, such as SR silicon rectifiers, SCR silicon-controlled rectifiers, GTR giant transistors, GTO thyristors, SITH static inductive thyristors, IGBT insulated gate bipolar transistors, PIN ultrahigh voltage diodes, smart power devices and power IC (Integrated Circuit) have more active demands on large-diameter zone-melted silicon single crystals, and therefore, the large-diameter zone-melted silicon single crystals have a broad application field and a favorable development prospect. A float zone melting method is adopted for the growth of the zone-melted silicon single crystals, that is to say, polycrystalline silicon materials are heated by adopting a high-frequency primary heating coil and are then molten, and the molten silicon is caught by seed crystals below the coil to continuously grow single crystal rods. By the adoption of float zone melting, the thermal field is concentrated near the melting zone, and the temperature distribution below the melting zone is not uniform. Especially for the growth of the large-diameter silicon single crystals, the surfaces of the single crystal rods are rapidly cooled, the radial temperature gradient of the center and the surface is enlarged, and thus the silicon single crystals are cracked. With the increase of the diameter of the single crystal rod, the melting zone area and the thermal stress are increased in a geometric level, and when the thermal stress of the single crystal rod is larger than its critical shear pressure, the single crystal rod shifts to cause broken ridges, and even rupture to further affect the production efficiency and damage equipment. In the existing technology, all large-diameter zone-melted single crystal silicon over 6.5 inches faces to the problem of single crystal rod cracking owing to unreasonable distribution of the thermal field and overlarge thermal stress, and broken ridges caused by unreasonable distribution of the thermal field and overlarge thermal stress in the growth process of 3-6 inch zone-melted single crystal silicon are one of the most major problems for improving the productivity of the silicon single crystals.
The zone melting furnace thermal field in the existing technology employs a single heat source, namely material feeding and melting are carried out by adopting a high-frequency power supply through a primary heating coil. It is difficult to adjust the heating process of a single power supply, and is especially difficult to control the thermal field distribution of a single crystal rod below the primary heating coil. In order to remedy the problem of insufficient heat of the single crystal rod below the melting zone and improve the distribution of the thermal field below the melting zone, a copper heat preservation ring device (refer to FIG. 1) is often adopted in the existing technology. The copper heat preservation ring device can reflect heat irradiated from the melting zone to the surface of the silicon single crystal to play a certain heat preservation role in the single crystal rod below the melting zone. However, the copper heat preservation ring device can only reflect the heat irradiated from the melting zone passively, and thus the reflected heat is uncontrollable in size and position. PRC (Disclosure) patents CN 102808216 A, CN102321913 A, CN102358951 A, CN202492612U and the like disclose a thermal field structure employing such primary heating ring and such heat preservation ring respectively, and different materials are also adopted for heat preservation in the disclosed technologies, however, these heat preservation devices employ passive heat preservation, are difficult to control and thus are inapplicable to growth of large-diameter zone-melted monocrystalline silicon. Hence, it is urgent to develop a zone melting furnace thermal field with a dual power heating function, so that the zone melting furnace thermal field distribution can be accurately controlled according to technological demands by virtue of reasonable control so as to solve the problem of large-diameter single crystal rod cracking and improve the quality of silicon single crystals.